Subsynchronous vibration

This example demonstrates the importance of the centered damper in suppressing the subsynchronous vibration exhibited by the relatively heavy rotor shown in Figure 6-27. This rotor was supported by o-ring... 

Richards, R.F., Vance, J.M., Paquette, D.J., Zeidan, F.Y., Using a Damper Seal to Eliminate Subsynchronous Vibrations in Three Back-to-Back Compressors, Proceedings of the 24th Turbomachinery Symposium, Texas A M University, p. 59, 1995. 

Figure 16-7 shows the vibration spectrum with the speed and suction pressure kept constant but with a small 20psig increase in discharge pressure. Notice the large increase in the 9000 rpm component from 0.2 to 1.5 mil (0.0127-0.0381 mm). A further small increase in discharge pressure would have increased the subsynchronous vibrations to more than 1.0 mil (0.0254 mm) and wrecked the unit. 

The overall vibration level on a machine is satisfactory for an initial or rough check. However, when a machine has a seemingly acceptable overall level of vibration, there may be hidden under this level some small levels of vibrations at discrete frequencies that are known to be dangerous. An example of this is subsynchronous instabilities in a rotor system. 

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